JP7360266B2 - imaging lens - Google Patents

Description

The present invention relates to an imaging lens that forms an image of a subject on a solid-state imaging device such as a CCD sensor or a C-MOS sensor used in an imaging device.

In recent years, various products such as automobiles, biometric authentication, and night vision monitoring have come equipped with camera functions that form images using near-infrared light. It is thought that the development of various products that combine camera functions will continue in the future.

Imaging lenses installed in such devices are required to have a wide angle and high resolution performance.

As a conventional imaging lens aiming at high performance, for example, an imaging lens as disclosed in Patent Document 1 below is known.

Patent Document 1 describes, in order from the object side, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, and a lens having a positive refractive power. An imaging lens is disclosed that includes a fourth lens and is configured such that the relationship between the focal length of the first lens at the s-line and the focal length of the entire system at the s-line satisfies a certain condition.

Unexamined Japanese Patent Publication No. 2018-13580

When attempting to widen the angle of view with the lens configuration described in Patent Document 1, it is extremely difficult to correct aberrations in the peripheral area, making it impossible to obtain good optical performance.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an imaging lens that satisfies the demand for a wide angle and has high resolving power with various aberrations well corrected.

Regarding the terms used in the present invention, the convex, concave, and flat surfaces of the lens are defined to refer to the shape near the optical axis (paraxial). Refractive power is defined as the refractive power near the optical axis (paraxial). A pole point is defined as a point on an aspheric surface other than on the optical axis where the tangential plane intersects perpendicularly to the optical axis. The optical total length is defined as the distance on the optical axis from the object side surface of the optical element located closest to the object side to the imaging surface. Note that the optical total length and back focus are distances obtained by converting the thickness of an IR cut filter, cover glass, etc. placed between the imaging lens and the imaging surface into air.

The imaging lens according to the present invention includes a first lens having a negative refractive power near the optical axis, which is arranged in order from the object side toward the image side, and a first lens having a negative refractive power near the optical axis, and a first lens having a positive or negative refractive power near the optical axis having a concave surface facing the object side. a second lens having a refractive power near the optical axis, and a third lens having a biconvex shape with a convex surface facing the object side and the image side near the optical axis and having a positive refractive power; and a fourth lens having positive refractive power.

In the imaging lens configured as described above, the first lens has negative refractive power to achieve a wide angle.

The second lens satisfactorily corrects comatic aberration, field curvature, and distortion. Furthermore, by oriented the concave surface near the optical axis toward the object side, distortion can be better corrected.

The third lens is designed to have a low profile, and also satisfactorily corrects spherical aberration, astigmatism, and distortion.

The fourth lens satisfactorily corrects coma, astigmatism, curvature of field, and distortion. Furthermore, by orienting the convex surface toward the image side near the optical axis, the angle of incidence of the light rays on the image sensor is appropriately controlled. As a result, the lens diameter of the fourth lens is reduced, making it possible to reduce the diameter of the imaging lens.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (1) be satisfied.
(1) 1.60<|r4|/T2
Here, r4 is the paraxial radius of curvature of the image-side surface of the second lens, and T2 is the distance on the optical axis from the image-side surface of the second lens to the object-side surface of the third lens.

Conditional expression (1) appropriately determines the relationship between the paraxial radius of curvature of the image-side surface of the second lens and the distance on the optical axis from the image-side surface of the second lens to the object-side surface of the third lens. It is specified within a certain range. By satisfying the range of conditional expression (1), the refractive power of the image-side surface of the second lens is maintained, and the angle of incidence of light rays on the object side of the third lens is appropriately suppressed, resulting in good distortion aberration. correction becomes possible.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (2) be satisfied.
(2) 2.10<r5/f<5.50
Here, r5 is the paraxial radius of curvature of the object-side surface of the third lens, and f is the focal length of the entire imaging lens system.

Conditional expression (2) defines the paraxial radius of curvature of the object-side surface of the third lens within an appropriate range. By satisfying the range of conditional expression (2), it becomes possible to satisfactorily correct spherical aberration, astigmatism, and distortion aberration.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (3) be satisfied.
(3) -1.40<f1/f4<-0.01
However, f1 is the focal length of the first lens, and f4 is the focal length of the fourth lens.

Conditional expression (3) defines the relationship between the focal length of the first lens and the focal length of the fourth lens within an appropriate range. By satisfying the range of conditional expression (3), it becomes possible to appropriately balance the refractive power of the first lens and the refractive power of the fourth lens. As a result, it becomes possible to effectively correct chromatic aberration, coma aberration, astigmatism, curvature of field, and distortion aberration.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (4) be satisfied.
(4) 2.85<D3/T3<12.00
However, D3 is the thickness of the third lens on the optical axis, and T3 is the distance on the optical axis from the image side surface of the third lens to the object side surface of the fourth lens.

Conditional expression (4) defines the relationship between the thickness of the third lens on the optical axis and the distance on the optical axis from the image side surface of the third lens to the object side surface of the fourth lens within an appropriate range. It is something to do. By satisfying the range of conditional expression (4), it is possible to reduce the height, appropriately suppress the angle of incidence of light rays on the object side of the fourth lens, and achieve good correction of coma, astigmatism, and distortion. becomes possible.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (5) be satisfied.
(5) -3.50<r3/|r4|<-0.10
However, r3 is the paraxial radius of curvature of the object-side surface of the second lens, and r4 is the paraxial radius of curvature of the image-side surface of the second lens.

Conditional expression (5) defines the relationship between the paraxial radii of curvature of the object-side surface and the image-side surface of the second lens within an appropriate range. By satisfying the range of conditional expression (5), it becomes possible to satisfactorily correct coma aberration, field curvature, and distortion aberration.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (6) be satisfied.
(6) 0.70<|r7|/f<5.50
However, r7 is the paraxial radius of curvature of the object-side surface of the fourth lens, and f is the focal length of the entire imaging lens system.

Conditional expression (6) defines the paraxial radius of curvature of the object-side surface of the fourth lens within an appropriate range. By satisfying the range of conditional expression (6), it becomes possible to satisfactorily correct coma aberration, astigmatism, and distortion aberration.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (7) be satisfied.
(7) 0.30<|r7|/(T3+bf)<2.20
However, r7 is the paraxial radius of curvature of the object-side surface of the fourth lens, T3 is the distance on the optical axis from the image-side surface of the third lens to the object-side surface of the fourth lens, and bf is the back focus. be.

Conditional expression (7) is based on the paraxial radius of curvature of the object-side surface of the fourth lens, the distance on the optical axis from the image-side surface of the third lens to the object-side surface of the fourth lens, and the back focus. It defines the relationship within an appropriate range. By satisfying the range of conditional expression (7), the refractive power of the object-side surface of the fourth lens is maintained, and the fourth lens is placed in an optimal position, making the aberration correction function of the lens more effective. Make it something. As a result, it becomes possible to effectively correct comatic aberration, astigmatism, field curvature, and distortion.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (8) be satisfied.
(8) -1.00<r2/r8<-0.10
However, r2 is the paraxial radius of curvature of the image-side surface of the first lens, and r8 is the paraxial radius of curvature of the image-side surface of the fourth lens.

Conditional expression (8) defines the relationship between the paraxial radius of curvature of the image-side surface of the first lens and the paraxial radius of curvature of the image-side surface of the fourth lens within an appropriate range. By satisfying the range of conditional expression (8), it becomes possible to satisfactorily correct coma aberration, astigmatism, field curvature, and distortion aberration.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (9) be satisfied.
(9) 8.00<(T3/f)×100<40.00
However, T3 is the distance on the optical axis from the image side surface of the third lens to the object side surface of the fourth lens, and f is the focal length of the entire imaging lens system.

Conditional expression (9) defines the distance on the optical axis from the image-side surface of the third lens to the object-side surface of the fourth lens within an appropriate range. By satisfying the range of conditional expression (9), it is possible to reduce the height, appropriately suppress the angle of incidence of the rays of light on the object side of the fourth lens, and enable good correction of astigmatism and distortion aberration. Become.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (10) be satisfied.
(10) 0.40<r2/f<2.50
However, r2 is the paraxial radius of curvature of the image-side surface of the first lens, and f is the focal length of the entire imaging lens system.

Conditional expression (10) defines the paraxial radius of curvature of the image-side surface of the first lens within an appropriate range. By satisfying the range of conditional expression (10), it becomes possible to satisfactorily correct distortion aberration.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (11) be satisfied.
(11) -6.50<r3/f<-1.40
However, r3 is the paraxial radius of curvature of the object-side surface of the second lens, and f is the focal length of the entire imaging lens system.

Conditional expression (11) defines the paraxial radius of curvature of the object-side surface of the second lens within an appropriate range. By satisfying the range of conditional expression (11), it becomes possible to satisfactorily correct coma aberration, field curvature, and distortion aberration.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (12) be satisfied.
(12) -5.20<r3/|r7|<-0.50
However, r3 is the paraxial radius of curvature of the object-side surface of the second lens, and r7 is the paraxial radius of curvature of the object-side surface of the fourth lens.

Conditional expression (12) defines the relationship between the paraxial radius of curvature of the object-side surface of the second lens and the paraxial radius of curvature of the object-side surface of the fourth lens within an appropriate range. By satisfying the range of conditional expression (12), it becomes possible to satisfactorily correct coma aberration, astigmatism, field curvature, and distortion aberration.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (13) be satisfied.
(13) 10.00<r5/T3<28.50
Here, r5 is the paraxial radius of curvature of the object-side surface of the third lens, and T3 is the distance on the optical axis from the image-side surface of the third lens to the object-side surface of the fourth lens.

Conditional expression (13) appropriately determines the relationship between the paraxial radius of curvature of the object-side surface of the third lens and the distance on the optical axis from the image-side surface of the third lens to the object-side surface of the fourth lens. It is specified within a certain range. By satisfying the range of conditional expression (13), the refractive power of the object-side surface of the third lens is maintained, the incident angle of the rays to the object side of the fourth lens is appropriately suppressed, and spherical aberration and non-uniformity are reduced. Good correction of point aberration and distortion becomes possible.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (14) be satisfied.
(14) -3.00<|r7|/r8<-0.10
However, r7 is the paraxial radius of curvature of the object-side surface of the fourth lens, and r8 is the paraxial radius of curvature of the image-side surface of the fourth lens.

Conditional expression (14) defines the relationship between the paraxial radii of curvature of the object-side surface and the image-side surface of the fourth lens within an appropriate range. By satisfying the range of conditional expression (14), it becomes possible to satisfactorily correct coma aberration, astigmatism, field curvature, and distortion aberration.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (15) be satisfied.
(15) r8/D4<-1.50
However, r8 is the paraxial radius of curvature of the image-side surface of the fourth lens, and D4 is the thickness of the fourth lens on the optical axis.

Conditional expression (15) defines the relationship between the paraxial radius of curvature of the image-side surface of the fourth lens and the thickness of the fourth lens on the optical axis within an appropriate range. By satisfying the range of conditional expression (15), it is possible to achieve a reduction in height and also to achieve good correction of coma aberration, astigmatism, curvature of field, and distortion aberration.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (16) be satisfied.
(16) -5.00<f1/f<-0.90
However, f1 is the focal length of the first lens, and f is the focal length of the entire imaging lens system.

Conditional expression (16) defines the focal length of the first lens within an appropriate range. By satisfying the range of conditional expression (16), it becomes possible to satisfactorily correct chromatic aberration and distortion.

Further, in the imaging lens having the above configuration, it is desirable that the following conditional expression (17) be satisfied.
(17) -1.20<f1/|f2|<-0.15
However, f1 is the focal length of the first lens, and f2 is the focal length of the second lens.

Conditional expression (17) defines the relationship between the focal length of the first lens and the focal length of the second lens within an appropriate range. By satisfying the range of conditional expression (17), it becomes possible to appropriately balance the refractive power of the first lens and the refractive power of the second lens. As a result, it becomes possible to effectively correct chromatic aberration, coma aberration, curvature of field, and distortion aberration.

According to the present invention, it is possible to obtain an imaging lens with high resolving power in which various aberrations are well corrected while satisfying the demand for a wide angle.

1 is a diagram showing a schematic configuration of an imaging lens according to Example 1 of the present invention. FIG. 3 is a diagram showing spherical aberration, astigmatism, and distortion of the imaging lens according to Example 1 of the present invention. FIG. 2 is a diagram showing a schematic configuration of an imaging lens according to Example 2 of the present invention. FIG. 7 is a diagram showing spherical aberration, astigmatism, and distortion of an imaging lens according to Example 2 of the present invention. FIG. 3 is a diagram showing a schematic configuration of an imaging lens according to Example 3 of the present invention. FIG. 7 is a diagram showing spherical aberration, astigmatism, and distortion of an imaging lens according to Example 3 of the present invention.

Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings.

FIG. 1, FIG. 3, and FIG. 5 each show a schematic configuration diagram of an imaging lens according to Examples 1 to 3 of the embodiment of the present invention.

The imaging lens of this embodiment includes a first lens L1 having a negative refractive power near the optical axis X, which is arranged in order from the object side toward the image side, and a concave surface near the optical axis a second lens L2 having a positive or negative refractive power and a third lens L3 having a biconvex shape with a convex surface facing the object side and the image side near the optical axis X and having a positive refractive power; The fourth lens L4 has a positive refractive power and has a convex surface facing the image side near the optical axis X.

Further, a filter IR such as an infrared cut filter and a cover glass CG are arranged between the fourth lens L4 and the imaging surface IMG (that is, the imaging surface of the image sensor). Note that this filter IR and cover glass CG can be omitted.

Since the aperture stop ST is disposed between the second lens L2 and the third lens L3, it facilitates size reduction in the radial direction.

The first lens L1 has negative refractive power to achieve a wide angle. The first lens L1 has a meniscus shape with a convex surface facing the object side near the optical axis X and a concave surface facing the image side. Therefore, spherical aberration and distortion are suppressed.

The shape of the first lens L1 is a biconcave shape with a concave surface facing the object side near the optical axis X and a concave surface facing the image side, as in Example 2 shown in FIG. Good too. In this case, better correction of chromatic aberration becomes possible.

The second lens L2 has negative refractive power and is formed in a biconcave shape with a concave surface facing the object side near the optical axis X and a concave surface facing the image side. Therefore, coma aberration, field curvature, and distortion are well corrected.

Note that the refractive power of the second lens L2 may be positive as in Example 3 shown in FIG. In this case, it is advantageous to reduce the height.

Further, the shape of the second lens L2 may be a meniscus shape with a concave surface facing the object side near the optical axis X and a convex surface facing the image side, as in Example 3 shown in FIG. good. In this case, coma aberration and distortion can be well corrected.

The third lens L3 has a positive refractive power and is formed in a biconvex shape with a convex surface facing the object side near the optical axis X and a convex surface facing the image side. Therefore, the height of the lens is reduced, and spherical aberration, astigmatism, and distortion are well corrected.

The fourth lens L4 has positive refractive power and is formed in a biconvex shape with a convex surface facing the object side near the optical axis X and a convex surface facing the image side. Therefore, the height of the lens is reduced, and coma aberration, astigmatism, curvature of field, and distortion are well corrected. In addition, by directing the convex surface toward the image side near the optical axis X, the incident angle of the light beam to the image sensor is appropriately controlled. As a result, the lens diameter of the fourth lens L4 is reduced, making it possible to reduce the diameter of the imaging lens.

In the imaging lens according to this embodiment, it is preferable that all of the first lens L1 to the fourth lens L4 are each composed of a single lens. A configuration with only a single lens can make extensive use of aspherical surfaces. In this embodiment, various aberrations are favorably corrected by forming appropriate aspheric surfaces on all lens surfaces. Furthermore, since the number of man-hours can be reduced compared to the case of using a cemented lens, it is possible to manufacture it at low cost.

In addition, the imaging lens according to this embodiment can be easily manufactured by using a plastic material for the lens, and mass production at low cost is possible.

Note that the lens material to be employed is not limited to plastic materials. By using glass materials, it is possible to aim for even higher performance. Although it is desirable that all lens surfaces be aspherical, spherical surfaces that are easy to manufacture may be used depending on the required performance.

The imaging lens in this embodiment achieves favorable effects by satisfying the following conditional expressions (1) to (17).
(1) 1.60<|r4|/T2
(2) 2.10<r5/f<5.50
(3) -1.40<f1/f4<-0.01
(4) 2.85<D3/T3<12.00
(5) -3.50<r3/|r4|<-0.10
(6) 0.70<|r7|/f<5.50
(7) 0.30<|r7|/(T3+bf)<2.20
(8) -1.00<r2/r8<-0.10
(9) 8.00<(T3/f)×100<40.00
(10) 0.40<r2/f<2.50
(11) -6.50<r3/f<-1.40
(12) -5.20<r3/|r7|<-0.50
(13) 10.00<r5/T3<28.50
(14) -3.00<|r7|/r8<-0.10
(15) r8/D4<-1.50
(16) -5.00<f1/f<-0.90
(17) -1.20<f1/|f2|<-0.15
however,
D3: Thickness of the third lens L3 on the optical axis X D4: Thickness of the fourth lens L4 on the optical axis X T2: Light from the image side surface of the second lens L2 to the object side surface of the third lens L3 Distance on the axis Focal length f2 of the first lens L1: Focal length f4 of the second lens L2: Focal length r2 of the fourth lens L4: Paraxial radius of curvature r3 of the image side surface of the first lens L1: On the object side of the second lens L2 Paraxial radius of curvature r4 of the surface: Paraxial radius of curvature r5 of the image side surface of the second lens L2: Paraxial radius of curvature r7 of the object side surface of the third lens L3: Paraxial radius of curvature of the object side surface of the fourth lens L4 Paraxial radius of curvature r8: Paraxial radius of curvature of the image-side surface of the fourth lens L4. Note that it is not necessary to satisfy all of the above conditional expressions, and by satisfying each conditional expression independently, Corresponding effects can be obtained.

Further, the imaging lens in this embodiment achieves more preferable effects by satisfying the following conditional expressions (1a) to (17a).
(1a) 2.00<|r4|/T2<40.00
(2a) 2.50<r5/f<4.40
(3a) -1.25<f1/f4<-0.03
(4a) 3.70<D3/T3<10.00
(5a) -2.60<r3/|r4|<-0.30
(6a) 1.10<|r7|/f<4.00
(7a) 0.45<|r7|/(T3+bf)<1.90
(8a) -0.65<r2/r8<-0.20
(9a) 13.00<(T3/f)×100<30.00
(10a) 0.65<r2/f<2.00
(11a) -5.80<r3/f<-2.10
(12a) -4.20<r3/|r7|<-0.75
(13a) 13.00<r5/T3<24.00
(14a) -2.00<|r7|/r8<-0.20
(15a) -18.00<r8/D4<-2.50
(16a) -3.50<f1/f<-1.40
(17a) -1.00<f1/|f2|<-0.25
However, the signs of each conditional expression are the same as those explained in the previous paragraph.

In this embodiment, the aspheric shape adopted for the aspheric surface of the lens surface has an axis in the optical axis direction of Z, a height in the direction perpendicular to the optical axis of H, a paraxial radius of curvature of R, a conic coefficient of k, When the aspheric coefficients are A4, A6, A8, A10, A12, A14, and A16, it is expressed by Equation 1.

Next, an example of the imaging lens according to this embodiment will be shown. In each example, f represents the focal length of the entire imaging lens system, Fno represents the F number, ω represents the half angle of view, ih represents the maximum image height, and TTL represents the total optical length. In addition, i is the surface number counted from the object side, r is the paraxial radius of curvature, d is the distance between lens surfaces on the optical axis (surface spacing), Nd is the refractive index of the d-line (reference wavelength), and νd is d The Abbe number for each line is shown. Note that an aspherical surface is indicated by adding an asterisk (*) after the surface number i.

(Example 1)

Basic lens data is shown in Table 1 below.

The imaging lens of Example 1 satisfies conditional expressions (1) to (17) as shown in Table 4.

FIG. 2 shows spherical aberration (mm), astigmatism (mm), and distortion aberration (%) for the imaging lens of Example 1. The spherical aberration diagram shows the amount of aberration for each wavelength of 800 nm, 850 nm, and 900 nm. In addition, the astigmatism diagram shows the amount of d-line aberration (solid line) on the sagittal image plane S, and the amount of d-line aberration (broken line) on the tangential image plane T (also in FIGS. 4 and 6). same). As shown in FIG. 2, it can be seen that each aberration is well corrected.

(Example 2)

Basic lens data is shown in Table 2 below.

The imaging lens of Example 2 satisfies conditional expressions (1) to (17) as shown in Table 4.

FIG. 4 shows spherical aberration (mm), astigmatism (mm), and distortion aberration (%) for the imaging lens of Example 2. As shown in FIG. 4, it can be seen that each aberration is well corrected.

(Example 3)

Basic lens data is shown in Table 3 below.

The imaging lens of Example 3 satisfies conditional expressions (1) to (17) as shown in Table 4.

FIG. 6 shows spherical aberration (mm), astigmatism (mm), and distortion aberration (%) for the imaging lens of Example 3. As shown in FIG. 6, it can be seen that each aberration is well corrected.

Table 4 shows the values of conditional expressions (1) to (17) according to Examples 1 to 3.

When the imaging lens according to the present invention is applied to a product having a camera function, it is possible to contribute to a wider angle of view of the camera and to improve the performance of the camera.

ST Aperture stop L1 First lens L2 Second lens L3 Third lens L4 Fourth lens ih Maximum image height IR Filter CG Cover glass IMG Imaging surface

Claims (6)

arranged in order from the object side to the image side,
a first lens having negative refractive power near the optical axis;
a second lens having positive or negative refractive power with a concave surface facing the object side near the optical axis;
a third lens having a positive refractive power and having a biconvex shape with a convex surface facing the object side and the image side near the optical axis;
and a fourth lens having positive refractive power with a convex surface facing the image side near the optical axis, and satisfying the following conditional expressions (1), (2), (3), and (9). Featured imaging lens.
(1) 1.60<|r4|/T2
(2) 2.10<r5/f<5.50
(3) -1.40<f1/f4<-0.01
(9) 8.00<(T3/f)×100<40.00
however,
r4: Paraxial radius of curvature of the image side surface of the second lens T2: Distance on the optical axis from the image side surface of the second lens to the object side surface of the third lens
T3: Distance on the optical axis from the image side surface of the third lens to the object side surface of the fourth lens
r5: Paraxial radius of curvature of the object-side surface of the third lens f: Focal length of the entire imaging lens system f1: Focal length of the first lens f4: Focal length of the fourth lens The imaging lens according to claim 1, characterized in that the following conditional expression (4) is satisfied.
(4) 2.85<D3/T3<12.00
however,
D3: Thickness of the third lens on the optical axis T3: Distance on the optical axis from the image side surface of the third lens to the object side surface of the fourth lens The imaging lens according to claim 1, characterized in that the following conditional expression (5) is satisfied.
(5) -3.50<r3/|r4|<-0.10
however,
r3: Paraxial radius of curvature of the object side surface of the second lens r4: Paraxial radius of curvature of the image side surface of the second lens The imaging lens according to claim 1, characterized in that the following conditional expression (6) is satisfied.
(6) 0.07<|r7|/f<5.50
however,
r7: Paraxial radius of curvature of the object-side surface of the fourth lens f: Focal length of the entire imaging lens system The imaging lens according to claim 1, characterized in that the following conditional expression (7) is satisfied.
(7) 0.30<|r7|/(T3+bf)<2.20
however,
r7: Paraxial radius of curvature of the object-side surface of the fourth lens T3: Distance on the optical axis from the image-side surface of the third lens to the object-side surface of the fourth lens bf: Back focus The imaging lens according to claim 1, characterized in that the following conditional expression (8) is satisfied.
(8) -1.00<r2/r8<-0.10
however,
r2: Paraxial radius of curvature of the image side surface of the first lens r8: Paraxial radius of curvature of the image side surface of the fourth lens